Many vehicles utilize automatic transmissions to transmit power from an internal combustion engine to the vehicle wheels. The automatic transmission adjusts the speed ratio between the engine and the wheels such that the engine can run at an efficient speed as the vehicle speed varies. At low vehicle speed, the transmission causes the wheels to rotate much slower than the engine while delivering a multiple of the engine torque to the wheels. At high speed, the transmission permits the engine to run at a relatively low speed even as the wheels rotate relatively fast. In reverse, the transmission delivers torque in the opposite direction causing the vehicle to move backwards even as the engine continues to rotate in the same direction.
The driver controls the transmission mode by manipulating a range selector. Range selectors typically provide Park, Reverse, Neutral, and Drive modes. In Park mode, the vehicle is held stationary even in the presence of substantial force. This is conventionally implemented by placing the transmission gearbox in a neutral state and holding the output shaft stationary with a parking pawl. In Reverse, the vehicle moves backward when the driver presses the accelerator pedal. In Neutral, pressing the accelerator pedal does not cause the vehicle to move although the vehicle is allowed to move in response to other forces such as gravity. Finally, in Drive, the vehicle moves forward when the driver presses the accelerator pedal. Reverse, Neutral, and Drive are conventionally implemented by placing the gearbox in a corresponding state with the parking pawl released.
When a driver moves the range selector from Park to Reverse or Drive. The gearbox must transition from a neutral state to a reverse gear ratio or a 1st gear ratio, respectively. In many automatic transmissions, this is accomplished by engaging one or more shift elements. Unfortunately, there may be a delay between the time the transmission begins engaging a shift element and the time at which the shift element reaches the required torque capacity to establish the reverse gear ratio or 1st gear ratio. The delay may be longer when the engine is running slowly. During this interval, the transmission is in neutral even though the driver has selected Reverse or Drive. If the driver presses the accelerator pedal during this interval, the engine speed will increase without causing the vehicle to move. If the vehicle is on a hill, it will begin to roll down the hill. Then, when the transmission finishes the engagement, the vehicle may lurch suddenly in the selected direction. If the driver is holding the brakes as the transmission finishes the engagement, the powertrain will begin to exert force against the brakes and the vehicle occupants may notice a jolt from the reaction torque at the transmission mounts.